THE WEAR OF GEAR TEETH WITH FUNCTIONALITY IN ABRASIVE RANDOMLY PARTICLES

Transcription

1 UPB Sci Bull, Series D, Vol 7, Iss, 10 ISSN THE WEAR OF GEAR TEETH WITH FUNCTIONALITY IN ABRASIVE RANDOMLY PARTICLES Monica VLASE 1, Andrei TUDOR Se arată că particulele abrazive ce pătrund în contactul convergent dintre doi dinţi ai unui angrenaj ce formează cupla de frecare, pot deteriora suprafaţa roţilor dinţate Uzura datorată particulelor abrazive depinde de modul în care are loc interacţiunea acestora atunci când geometria particulelor este aleatoare Se evaluează numărul de particule din interstiţiul dintre doi dinţi ai angrenajuluise determină uzura ca parametru determinist aleator folosind modelele de deformare corespunzătoare Se consideră că particulele abrazive sunt ovoide cu lungimea razelor R şi r variabile aleatoare Distribuţia variabilă aleatoare este normală şi exponenţială It is shown that the abrasive particles, which penetrate in the convergent joint of gear s teeth friction pairs, can damage the superficial layer of surfaces The wear caused by individual particle depends on having both a satisfactory understanding of individual interactions and a suitable procedure for combining these under circumstances when particle geometry has randomly aspects The particle number in the gap of gear teeth is evaluated Wear determinist and random parameter predicted from the corresponding particle deformation models are shown It is considered that the abrasive particles are ovoid with random variable radii R and r The random variable distributions are considered to be exponential and normal Keywords: abrasive wear, gear teeth, random geometry, durability 1 Introduction Friction couples operating in infested environments with abrasive particles are one of durability restriction case For the convergent-divergent gaps (interstices), with relative surfaces motion, the particles penetration and their driving into the gap, depend on the gap s geometry, the particle geometry and friction coefficient The harder particles can appear from the exterior couple environment (dust, sand) or even from the wear particle detachment in the adhesion or fatigue process [1,,3,4] Abrasion can occur due to: micro-cutting by the hard particle sharp edges or their roughness; by fracturing due to the crack convergence; by fatigue due to repeated plastic streins; by hard granule, plucking out of the material [3,4] 1 Lecturer, Technical University of Civil Engineering of Bucharest, Romania Professor, University POLITEHNICA of Bucharest, Romania, tudor@mecaomtrpubro

2 154 Monica Vlase, Andrei Tudor The way in which the abrasive particles pass on the worn surface shows two types of wear: - two elements wear; - three elements wear Two elements abrasion supposes that particles are fixed in one of the couple elements This can be considered similar to the abrasion of one surface by the roughness of the conjugated surface [1,,3] Gear mechanisms can work in abrasive environments as open gears or due to the sealing failure The goal of this paper is to analyse the abrasive wear produced by ovoid particles with aleatory variable geometry, into a gear teeth gap Contact model between an ovoid and a cylinder Abrasive particles attach to the gear teeth flanks, as shown in Fig1 Fig1 Abrasive particles attached to the gear tooth flank In order to model the wear phenomenon between the abrasive particles and gear tooth flank, the abrasive particle geometry is idealised as an ovoid, characterised by two spherical segments of radii R and r and the distance a p between the spheres centre This ovoid is showen in Fig R Fig Geometry of the ovoid abrasive particle The following dimensionalless values are defined: length coefficient ξ = a p / R rounding coefficient χ = r / R

3 The wear of gear s teeth with functionality in abrasive randomly particles 155 particle relative radius R ap = R / Rc The gap (interstice) between the gear teeth is variable, depending on the contact point on the gearing line, as showen in Fig3 Ovoid into the gear s gap Gearing line O 1 x C O Fig3 Mobil contact point on gearing line The initial hypothesis is that the gear teeth deformations are elastic and the ovoid particles are perfectly rigid The contact of teethes will be considered as a contact between an equivalent cylinder with a rigid planethe equivalent cylinder is defined similar to Hertz solution of the contact pressure In this case, the contact between ovoid and teeth will be considered a contact between the elastic or plastic cylinder and the rigid ovoid We consider that the ovoid particle is in contact with one tooth through the greater sphere and in contact with the next tooth through the smaller sphere of this ovoid and the direction of the ovoid is radial To generalize the results, the geometric parameters of the gear (rolling radius, base circle radius, gearing line length) are made dimensionless by the gear modulus It is considered that the teeth number of the drive gear, (z 1 ), and the transmision ratio, (i t ), are known In this paper, we analyse only the case of spur gear teeth The corelation between the gear geometry and ovoid geometry is based on the

4 156 Monica Vlase, Andrei Tudor ovoid greather sphere radius, (R), gear modulus, (m), and dimensionless radius of ovoid (R am =R/m) To define the contact between gear tooth and the ovoid particle it is necessary to determine the number of the abrasive particles which exists in the gap Using the geometric parameters of the teeth and the ovoid, we can calculate this number per unit teeth length (z p ): βmax βmin z = + 1 (1) p βc where: β max is the maximum contact angle between the tooth and the greater sphere of the ovoid (wrapping angle of equivalent cylinder); β min is the minimum contact angle between the tooth and the small sphere of the ovoid; β c is the angle between two consecutive ovoids in contact Figs 4 and 5 present the abrasive particle number variation, (z p ), versus point position on the line of action, (x-dimensionless lenght of contact point relative to the pitch point), for different values of the length coefficient, (ξ), and the rounding coefficient, (χ) of abrasive particle The pitch point C (figure 3) is considered as an origin for the contact point between teeth (figures 4 and 5) Fig 4 The contact angle between tooth (equivalent cylinder) and ovoid The ovoid positions into the gap between the gear teeth can be random, depending on the geometry of abrasive particle and equivalent cilinder

5 The wear of gear s teeth with functionality in abrasive randomly particles zp ξ=1 10 ξ=4 ξ= z 1 =; i t =; m=3; R am =10-4 ; χ= Fig5 The variation of the abrasive particle x number, (z p ), versus contact point on the line of action (x), for different values of the length coefficient, (ξ) zp χ=0 χ=08 χ=05 z 1 =; i t =; m=3; R am =10-4 ; ξ= x Fig6 The variation of the abrasive particle number, (z p ), versus point position on the gearing line, (x), for different values of the rounding coefficient, (χ) z 1 Figs 7 and 8 present the variation of the abrasive particle number, (z p ) versus abrasive particle relative dimension, (R am ), for different values of the length coefficient, (ξ), and the rounding coefficient, (χ) The external F force distribution upon the abrasive particles z p is determined using the following simplifying hypothesis: - the cylinder strains are elastic; - the abrasive particles are rigid, thus, the distribution circle of the ovoid big spheres centers is concentric with the defined circle of the cylinder specific to the friction couple; - the abrasive particles number on the contact unit length of the couple is elevated (10 4 order of magnitude) and the wrapping angle is small ( ), thus, it is accepted that the abrasive particles number, (z p ), loaded by the external force (F) is a (β) contact angle continuous function

6 158 Monica Vlase, Andrei Tudor ξ= ξ=4 z 1 =; i t =; m=3; x=0; χ=05 zp ξ= Fig7 The variation of the abrasive particle number, (z p ), versus abrasive particle relative dimension, (R am ), for different values of the length coefficient, (ξ) R am χ=1 χ=05 z 1 =; i t =; m=3; x=0; ξ= zp χ= R am Fig8 The variation of the abrasive particle number, (z p ), versus abrasive particle relative dimension, (R am ), for different values of the rounding coefficient, (χ) It is known that between the force in a certain point, situated at the angular distance β and the elastic strain, there is a hertzian relation as: 3/ F β = cδ () where: c is the hertzian rigidity in the specific point, being accepted as constant all along the circular contact length of the cylinder with the hard particles; δ is the strain law The specific force can be deduced if the strain law, (δ), is known For the accepted case, with the greather spheres centers of the ovoid situated on a circle, the following expression is proposed: δ = A tan( β + γ ) (3) where the constants A and γ can be determined from the limit conditions: δ = δ1 for β = β min

7 The wear of gear s teeth with functionality in abrasive randomly particles 159 Thus, and δ = 0 for β = β max A = tan δ ( β β ) min 1 φ = β max max From the condition of mechanical equilibrium, 1 F a1= 3 z p tan max min ( 1) cos β β i β c β 3 i= 1 tan( β max β min ) [ ] [ + ( i 1) β ] min c (4) The maximum non-dimensional elastic force (F a1 = F 1 /F) supported by the cylinder is determined Thus, Figs 9 and 10 present the variation of the maximum dimensionless force versus relative dimension of the abrasive particle for different values of the length coefficient, (ξ), and rounding coefficient, (χ) ξ= Fa1 016 ξ=3 008 ξ=4 z 1 =; i t =; m=3; x=0; χ= R am Fig9 The variation of the maximum dimensionless force, (F a1 ), versus relative dimension of the abrasive particle, (R am ), for different values of the length coefficient, (ξ) Fig 9 shows that the maximum dimensionless force, (F a1 ), from the abrasive particle-equivalent tooth contact decreases with the increase of the abrasive particle length coefficient (ξ)

8 160 Monica Vlase, Andrei Tudor Fa z 1 =; i t =; m=3; x=0; ξ= χ=01 χ=05 χ= Fig10 The variation of the maximum dimensionless force, (F a1 ), versus relative dimension of the abrasive particle, (R am ), for different values of the rounding coefficient, (χ) Fig 10 shows that the rounding coefficient, (χ), has an important effect on the force distribution in the abrasive particle-equivalent tooth contact area 3 The elastic loading capacity of the cylinder In order to evaluate the elastic loading capacity between the rigid, ovoid particle and the elastic cylinder contact one shall use the Tresca plasticity criteria [,3] I T = max( σ1 σ, σ σ 3, σ 3 σ1 ) (5) where: σ 1, σ, σ 3 are the principal dimensionless stresses The condition for flow avoidance, evaluated with axial flow strength σ c, is: po 1 σ c IT where p o is the pressure in the center of hertzian contact The elastic loading capacity of the sphere-rigid ovoid contact can be defined as p ratio o For the function characteristics (friction coefficient f, contact angle β) σ c p situated on the 1/I M curve, the elastic loading capacity o can be evaluated For σ c the function characteristics situated above the 1/I T curve, the deformation states is plastic and the stresses and deformation state can be determined with the plasticity theory For the function characteristics situated beneath the 1/I T curve, the wear particle appearance is defined by the elastic fatigue after a specific cycle number (Wöhler curve type) R am

9 The wear of gear s teeth with functionality in abrasive randomly particles 161 Regarding the critical deformations of the transition from the elastic state to the plastic one, one considers the penetration (the interference) of the p o pressure, for which the Tresca invariant reaches the limit value Thus, using the Hertz equation system one can obtain: h π p ( ) o π σc Eo h = = = = acr hk (6) R cr 1 Eo 1 IT where h is maximum interference (penetration) of ovoid to the elastic cylinder, R- radius of equivalent cylinder, E o elasticity modulus of cylinder For higher interference between abrasive particle and gear s teeth the deformations are plastic and they are determined with the Hencky slip-lines When operating, due to working conditions, abrasive particles (rigid ovoids) occur, then these particles can create plastic deformations on the counter piece (cylinder or sphere) These plastic deformations occur when the equivalent stress evaluated by the von Mises or Tresca parameter equalizes or becomes grater than the counter piece yield limit The elastic loading capacity is defined that the maximum contact pressure (p max ) for which the elastic limit of the material occurs We consider that dimensionless parameter (p omax ) as a ratio of the maximum contact pressure and the yield limit of material The durability of friction pair with elastic strain can be evaluated using the Wohler fatigue curve The rigid abrasive particle has a plastic has a plastic penetration into gear teeth for the first contact and until the area contact becomes large, the deformation are elastic [4, 5] 4 Wear model for plastica contact To evaluate the contact surface of a rigid particle with a plastically plane deformed, the Hencky slip-line method is used The plastic strain state is considered The ovoid particles from the gap determine different contact angles with the cylinder The particles effects upon the strain state are determined by the adhesion angle of abrasive particle to the tooth flank, defined by friction angle ( f) and attack angle of the abrasive particle (α) [6,7,8] The following angles are defined, which are specific to Hencky strain lines: ε = 05 acos() f ; φ = α ε ; sin( α ) η = a sin 1 f For the specific conditions of the contact between a small sphere (micrometric radius) and a cylinder (millimeter radius), the wear process is determined by the Archard coefficient

10 16 Monica Vlase, Andrei Tudor Depending on the cutting (α) and adhesion (ε) angles, two cases can be analyzed When the cutting angle α < ε, the Archard wear parameter has the expression [5] 3 γ p sin π ka1 = η f γ r f an ( ε) sin( α) + 05 sin( α) cos( α) + sin( α) tan ( 7) where: f = A sin( α ) + cos( ε α ) f = A cos( α ) + sin( ε α ) 1 n n π A n = ε α η The shearing deformation f1 sin( φ ) γ = f sin( ε ) sin( ε ) sin( α ) and yield strain by shear stress Em γ c = with the melting energy per unite of volume (E m ), and yield limit (τ c ) τ c The Archard wear parameter, for the cutting angle α > ε is ( 05 sin( α ) + sin( α ) ) 3 05 γ ka = f γ (8) c Fig 11 shows the Archard wear parameter as a function of the cutting angle of abrasive particle for the yield strain corresponding to three materials 1 01 γ r = 10 γ r = 8 γ r = 6 ka f = 0 p an = α Fig 11 Archard wear parameter vs cutting angle of the abrasive particle When the cutting angle of the abrasive particle or other parameter is a random variable, it is necessary to analise the effect of probabilistic parameter on the wear parameter or friction coefficient []

11 The wear of gear s teeth with functionality in abrasive randomly particles 163 We define the wear random parameter (I w ), x I w = max k A ( x1, x,, xn ) f p ( x) xm dx (9) xmin where: k A (x 1,x,,x n ) is the Archard deterministic wear parameter for the x 1, x,, x n variable; f p (x) frequency function of stochastic x variable; x m statistic media of x variable; x min, x max minimum and maximum of stochastic x variable In this paper, we apply this concept to the cutting angle, that a stochastic variable The exponential and normal probabilistic laws appear in some tribological phenomena [,7,8] a) The exponential law The frequency function of cutting angle (α) is f e ( α ) = exp ( α B) (10) where the constant B will be evaluted by the integrated law of frequency function (the sume of all probabilistic events is 100%) Fig 1 shows the effect of friction coefficient on the wear random parameter, for some dimensionless nominal contact pressure, when the yield strain is γ c =10 Fig 13 shows that the contact pressure increases proportional to the random wear parameter Iwe γ c =10 α max = 3 o α med = 03 o p a = 05 p a = 005 p a = f Fig1 Wear exponential random parameter vs adhesion parameter The wear parameter decreas or increas, when the statistic parameters of cutting angle are variable For example, Fig 13 shows the effect of variation of the maximum cutting angle, when the minimum angle is constant

12 164 Monica Vlase, Andrei Tudor Iwe f = 01 f = 0 f= p a Fig13 Wear exponential random parameter vs dimensionless contact pressure The wear parameter decreas or increas, when the statistic parameters of cutting angle are variable For example, Fig 13 shows the effect of variation of the maximum cutting angle, when the minimum angle is constant Iwe p a = α max, o f=01 γ r = 10 α m =0 p a = 01 Fig14 Effect of maximum statistic angle about wear random parameter b) The Gauss normal law The frequency function of cutting angle (α) is 1 α α ( ) m f α = exp (11) pn π σ α when σ α is standard deviation of cutting angle We accept the rule of the 6 th σ α for the maximum and minimum statistic cutting angle, when the media and standard deviation are known α = α m 3σ α α α 3σ α min max = m +

13 The wear of gear s teeth with functionality in abrasive randomly particles 165 Thus, for example, the wear random parameter as a function to the adhesion parameter and the contact pressure are shown in Figure 15 and Figure 16 IwN γ r = 10 σ σ = 003 p a = 0 p a = p a = f Fig15 Wear normal Gauss random parameter vs adhesion parameter γ r = 10 σ σ = 003 f = 015 f = 0 IwN f = p a Fig16 Wear normal Gauss random parameter vs contact pressure The effect of standard angle deviation (σ α ) about the Archard wear parameter is shown in Figure I un1 () σ IWN I un () σ I un3 () σ p a = 01 γ r = 10 f = σ α Fig 17 Wear normal Gauss random parameter vs standard angle deviation

14 166 Monica Vlase, Andrei Tudor It can be seen that the Arhard wear parameter increases verry rapidly with the standard deviation If the wear random parameter is known, it is possible to estimate the durability of the friction pairs, function to geometric aspects 5 Conclusions Abrasion due to micro-cutting by the hard particle sharp edges or its roughness is one important forme to limits the durability of friction pairs The abrasive particle is considered to be an ovoid, characterised by two spherical segments and the distance between the spheres centre The ovoid particles number is a very important parameter for the wear phenomena of gears and it depends on the teeth number of the driving gear and the gear ratio We developed a theoretical model of the maximum dimensionless elastic load supported by the cylinder in contact with abrasive ovoid For higher penetration (interference) values, the deformations are plastic and we apply the Hencky slip- lines and thus, we obtained the Archard wear coefficient as a function on the cutting angle and the adhesion coefficient The exponential and normal probabilistic laws are exemplified to analyze the wear parameter The random wear coefficient is predicted and can be used to determine the durability of friction pairs R E F E R E N C E S [1] VA Ikramov, Rascetni metod otenki abrazvovo iznosa, (Calculus Methods of Abrasive Wear Evaluation) Moskva, Masinostroenie, 1987 [] A Tudor, Contactul real al suprafeţelor cuplelor de frecare, (Real Contact of Friction Pairs Surfaces) Bucureşti, Editura Academiei Române, 1990 [3] K Kato, Micro-mechanisms of wear- wear modes, Wear 153, 199, p77-95 [4] Y Xie, JA Williams, The generation of worn surfaces by the repeated interaction of parallel grooves, Wear , 1993, p [5] D Maugis, Contact, adhesion and rupture of elastic solid, Springer, 00 [6] EM Kopalinsky, Oxley, PLB, Explain the mechanics of metallic sliding friction and wear in terms of slip line field models of asperity deformation, Wear 190, 1995, p [7] P Lacey, AA Torrance, The calculation of wear coefficient for plastic contact, Wear 145, 1991, p [8] AA Torrance, TR Buckley, A slip-line field model of abrasive wear, Wear 196, 1996, p35-45